Biofuels for transportation are attractive replacements for gasoline and are rapidly penetrating fuel markets as low concentration blends. Biofuels, derived from natural plant sources, are more environmentally sustainable than those derived from fossil resources (such as gasoline), their use allowing a reduction in the levels of so-called fossil carbon dioxide (CO2) gas that is released into the atmosphere as a result of fuel combustion. In addition, biofuels can be produced locally in many geographies, and can act to reduce dependence on imported fossil energy resources. Alcohols suitable for use as biofuels include ethanol, butanol and 2,3-butanediol.
Ethanol is rapidly becoming a major hydrogen-rich liquid transport fuel around the world. Worldwide consumption of ethanol in 2002 was an estimated 10.8 billion gallons. The global market for the fuel ethanol industry is also predicted to grow sharply in future, due to an increased interest in ethanol in Europe, Japan, the USA and several developing nations.
Butanediols including 1,2-butanediol, 1,3-butanediol, 1,4-butanediol and 2,3-butanediol may be considered to have a variety of advantages over ethanol. Like ethanol, butanediols may be used directly as an automotive fuel additive. They may also be relatively easily transformed into a number of other potentially higher value and/or higher energy products. For example, 2,3-butanediol may be readily converted in a two step process into an eight-carbon dimer which can be used as aviation fuel.
2,3-butanediol derives its versatility from its di-functional backbone, i.e., 2 hydroxyl groups are located at vicinal C-atoms allowing the molecule to be transformed quite easily into substances such as butadiene, butadione, acetoin, methylethyl ketone etc. These chemical compounds are used as base molecules to manufacture a vast range of industrially produced chemicals.
In addition, 2,3-butanediol may be used as a fuel in an internal combustion engine. It is in several ways more similar to gasoline than it is to ethanol. As the interest in the production and application of environmentally sustainable fuels has strengthened, interest in biological processes to produce 2,3-butanediol (often referred to as bio-butanol) has increased.
Carbon Monoxide (CO) is a major by-product of the incomplete combustion of organic materials such as coal or oil and oil derived products. Although the complete combustion of carbon containing precursors yields CO2 and water as the only end products, some industrial processes need elevated temperatures favouring the build up of carbon monoxide over CO2. One example is the steel industry, where high temperatures are needed to generate desired steel qualities. For example, the steel industry in Australia is reported to produce and release into the atmosphere over 500,000 tonnes of CO annually.
Furthermore, CO is also a major component of syngas, where varying amounts of CO and H2 are generated by gasification of a carbon-containing fuel. For example, syngas may be produced by cracking the organic biomass of waste woods and timber to generate precursors for the production of fuels and more complex chemicals.
The release of CO into the atmosphere may have significant environmental impact. In addition, emissions taxes may be required to be paid, increasing costs to industrial plants. Since CO is a reactive energy rich molecule, it can be used as a precursor compound for the production of a variety of chemicals. However, this valuable feedstock has not been utilised to produce 2,3-butanediol.
It has been demonstrated that 2,3-butanediol can be produced by microbial fermentation of carbohydrate containing feedstock (Syu M J, Appl Microbiol Biotechnol 55:10-18 (2001), Qin et al., Chinese J Chem Eng 14(1):132-136 (2006)). 2,3-butanediol may also be produced by microbial fermentation of biomass from crops such as sugar beet, corn, wheat and sugarcane. However, the cost of these carbohydrate feed stocks is influenced by their value as human food or animal feed and the cultivation of starch or sucrose-producing crops for 2,3-butanediol production is not economically sustainable in all geographies. Therefore, it is of interest to develop technologies to convert lower cost and/or more abundant carbon resources into 2,3-butanediol.
Production of 2,3-butanediol by microbial fermentation of gaseous substrates comprising CO has been demonstrated. However, the production of 2,3-butanediol by these processes has been a secondary product. Production of other products including ethanol is favoured in fermentation. Butanediol has greater value than the other products produced in such fermentations. It is desirable to be able to affect the fermentation in such a way that the production of 2,3-butanediol is increased. It has previously been shown that increased 2,3-butandiol productivity was influenced by a rate of hydrogen consumption by a microbial culture (WO2012131627).
There remains a need on the art to increase the ability to produce valuable products from industrial gaseous substrates in economically beneficial ways. There is a need to enhance the production of 2,3-butanediol relative to the production of other products that are routinely produced in the fermentation of gaseous substrates by carboxydotrophic bacteria.